This invention relates to novel hydroxypropyl substituted granular starch materials having a pasting temperature reduced from that of the granular starches before hydroxypropyl substitution. The pasting temperature of the present starch materials is substantially below the pasting temperature of hydroxypropyl substituted granular starches, having like levels of hydroxypropyl substitution, which are known in the art.
It is well known in the art that granular starch materials can be chemically modified by reaction with alkylene oxides and alkyl halides to provide granular starch ethers characterized by pasting temperatures below that of the parent unmodified granular starch. Indeed by controlling the amount of hydroxyalkylating agent and the conditions of reaction the skilled practitioner can prepare etherified granular starch materials having a pasting temperature ranging from just below the pasting temperature of the parent unmodified granular starch to just above the freezing point of water. Typically the greater the degree of substitution (hydroxyalkylation) the greater the observed reduction in pasting temperature of the hydroxyalkylated modified granular starch.
Hydroxyalkyl starch ethers are prepared commonly by the reaction of granular starch materials with ethylene oxide, propylene oxide or epichlorohydrin preferably in the presence of an alkali metal hydroxide catalyst. That reaction has been conducted under a wide variety of process conditions under which the hydroxyalkylated starch product material remains in its native ungelatinized granular form. In that form it is easily dewatered and washed and thus economically produced in a relatively pure state.
The selection of reaction conditions for conducting hydroxyalkylation of granular starches is based somewhat on the level of hydroxyalkyl substitution desired in the product starch material. Where a degree of substitution (D.S.) of up to about 0.1 hydroxyalkyl groups per anhydroglucose unit in the starch polymer is desired, a conventional aqueous reaction process can be employed. This is the process most widely used commercially for the production of reduced pasting temperature hydroxyalkylated starches in the United States. In the aqueous process, granular starch in a 40 to 45% solids suspension is made alkaline with an alkali metal hydroxide or alkaline earth metal hydroxide; ethylene or propylene oxide is added to the suspension. The reaction is conducted at temperatures well below the swelling temperature of the starch usually not exceeding 50.degree. C. Often swelling inhibitors such as neutral alkali metal salts or water miscible organic solvents are added to the reaction mixture to prevent the swelling of the granular starch material under the alkaline reaction conditions. The alkali is added to the reaction mixture to promote efficient and rapid reaction of the starch with the alkylene oxide reagents. U.S. Pat. No. 2,516,633 describes such a process for reacting unswollen granular starch under non-swelling conditions with an alkylene oxide in a water solution of a water soluble alkali metal or alkaline earth metal salt.
As alluded to hereinabove, because of granular swelling, it is difficult to prepare ungelatinized, hydroxyalkyl substituted starch products containing more than about 0.1 hydroxyalkyl groups per glucose unit when reactions are conducted in water suspensions. In order to obtain higher degrees of substitution, it is necessary to conduct the etherification in the presence of a limited amount of moisture. Such has been accomplished by reaction either in the absence of a liquid reaction medium (dry state) or in a liquid such as a lower aliphatic alcohol which has little or no tendency to swell the product hydroxyalkyl starch ethers. There are a number of U.S. patents directed to starch hydroxyalkylation processes which eliminate or at least minimize the exposure of the product hydroxyalkyl starches to moisture thereby facilitating isolation of higher-substituted hydroxyalkyl starches.
U.S. Pat. Nos. 2,516,632 and 2,516,634 each describe a process which involves the reaction of alkylene oxide in the gaseous form with ungelatinized granular starch material. The U.S. Pat. No. 2,516,632 describes the direct reaction of gaseous alkylene oxides with granular starch by agitating a mixture thereof in a closed vessel until substantially no free alkylene oxide remains. The U.S. Pat. No. 2,516,634 describes a similar process except a soluble alkali metal salt is first dispersed in the granular starch prior to bringing the starch into contact with the reactive alkylene oxide.
There is also a substantial number of patents which disclose the hydroxyalkylation of granular ungelatinized starch in substantially non-aqueous liquid media in the presence of alkaline catalyst. U.S. Pat. No. 2,682,535 describes a process for producing a hydroxyalkyl ether of a polysaccharide, including celluloses and starches, by reacting polysaccharide with an alkylene oxide in the presence of caustic alkali in a mixture of a C.sub.4-10 monohydric alkanol and a 2-(C.sub.1-4 alkoxy) isopropanol at a temperature no higher than 50.degree. C. U.S. Pat. No. 2,744,894 discloses a similar process wherein the liquid phase of the reaction medium is comprised of a mixture of a high boiling (85.degree. C.+) organic liquid and a C.sub.1-4 alkoxy ethanol or C.sub.1-4 alkoxyisopropanol.
U.S. Pat. No. 3,127,392 discloses a process for preparing hydroxyalkyl amylose by reacting amylose with an alkylene oxide under alkaline conditions in a polar solvent including water and alcohols at temperatures between 30.degree. C. and 90.degree. C. for approximately 1 to 24 hours. U.S. Pat. No. 3,135,739 describes a process for preparing a highly substituted starch ether in the granular ungelatinized state by reacting moist alkaline granular starch in an aliphatic ketone with an alkylene oxide until the starch contains in excess of 0.5 ether groups per anhydroglucose unit. The starch products from that process swell in cold water.
U.S. Pat. No. 3,014,901 also describes a process for preparing ungelatinized ethers wherein the granular starch is first reacted with an "inhibiting amount" of epichlorohydrin and thereafter with ethylene oxide in an aqueous alkaline slurry until the degree of substitution of the granular starch product lies within the range of 0.15 to 0.55.
U.S. Pat. No. 2,845,417 discloses the process for preparing ungelatinized hydroxyalkyl derivatives of granular starch which comprises forming a slurry of granular starch in water, a water miscible alcohol, and an alcohol soluble alkali, and then reacting the granular starch in said slurry with a hydroxyalkylating agent to provide a partially etherified ungelatinized product in filterable form. The reaction is conducted at a temperature between about 60.degree. and 130.degree. F. for a period of up to about 24 hours. Under the processing conditions therein disclosed, the amount of hydroxyalkylating agent may be adjusted to give products which will swell or gelatinize in water at any desired temperature between the normal gelatinization temperature of the granular starch and the freezing point of water.
U.S. Pat. No. 4,015,067 also describes a hydroxyalkylation process and focuses on the preparation of cellulose-polysaccharide ethers by (1) preparing a slurry of cellulose, alkali metal hydroxide, and an etherifying ingredient with or without an inert carrier fluid (including an excess of alkyl chloride, dimethyl ether, hexane, benzene or toluene) in the substantial absence of free oxygen; (2) feeding the slurry into a heated tubular reactor at a pressure of about 100 to about 400 psig at a rate to provide a reaction period of 10 to 60 minutes at 30.degree. to 135.degree. C.; and (3) recovering the polysaccharide ether from the slurry.
In sum the prior art describes essentially three general types of processing conditions for the preparation of hydroxyalkyl granular starch ethers by the reaction of granular starch and alkylene oxides:
(1) In an aqueous medium in the presence of an alkali metal hydroxide base and optionally in the presence of swelling-suppressing salts;
(2) In the absence of any reaction medium, that is, the direct reaction of alkylene oxide in the gaseous or liquid state with a granular starch material, optionally in the presence of an alkali metal hydroxide or alkali metal salt; or
(3) In a substantially non-aqueous, alkaline medium.
Those skilled in the art will appreciate that regardless of which method is used for the preparation of hydroxyalkyl starch ethers, generally there is a direct relationship between the level of hydroxyalkyl substitution and the observed reduction in pasting temperature; the higher the level of hydroxyalkyl substitution the lower the pasting temperature exhibited by the modified granular starch. The pasting temperature exhibited by hydroxypropyl substituted starches is typically reduced from the pasting temperature of the unmodified starch by about 2.degree. up to about 5.degree. C. for each 1% by weight of hydroxypropyl content (or for each 0.025 D.S.).
Because of their reduced pasting temperature, hydroxypropyl substituted granular starches find broad application as thickeners in quick-cooking food formulations such as gravies, sauces, pie fillings, etc. Those skilled in the art will recognize the inherent advantage which would be realized by a hydroxypropylated, low-pasting-starch having a degree of substitution (D.S. hydroxypropyl) substantially lower than that of presently available hydroxypropylated food starches of similar pasting temperatures. Not only would the cost of hydroxypropylating agent be reduced, but also there would be inherent advantage perceived by the consumer in minimizing the level of chemical modification of starch ingredients in the many starch containing food compositions which are included in the consumer's diet.
Accordingly it is an object of the present invention to provide hydroxypropylated reduced pasting-temperature food starches which have a pasting temperature characteristic of conventional hydroxypropylated starches having much higher levels of hydroxypropyl substitution.
It is a further object of this invention to provide novel granular starch products which have gelatinization temperatures ranging from that of cold water, that is, about 50.degree. F. (10.degree. C.), to temperatures within about 10.degree. of the pasting temperature of the parent unsubstituted granular starch with hydroxypropyl substitution levels as little as one-half the levels normally required to achieve equal reductions in pasting temperature.
Another object is to provide a process for the preparation of novel hydroxypropylated granular starches which exhibit a pasting temperature reduced from that of the granular starch before hydroxypropylation by at least 6.5.degree. C. for each 1% by weight hydroxypropyl substitution.
The above and related objects are achieved in the present invention by an improved hydroxypropyl substituted granular starch prepared by reacting granular starch in a lower alkanol with propylene oxide under alkaline conditions at high temperatures, that is, temperatures above 100.degree. C. The hydroxypropylated granular starch products in accordance with this invention exhibit unexpectedly low pasting temperatures at hydroxypropyl substitution levels between about 2 and about 9% by weight of the starch. The present improved hydroxypropyl substituted granular starch can be used as a thickening agent in foods or food ingredients which require the functionality of a reduced-pasting-temperature granular starch component.